What are your end goals? Do you ultimately want to take them both into Heroics or Raids? Are you just going to be doing PvE questing. Do you have any PvP aspirations? Why do you want to do "mass AoE farming"? All of these play a factor.

My thoughts, if you're just doing PvE and want to level, and perhaps venture into some Heroics (with friends, guildmates, whatever), I would stick with your DK and add either a Hunter or an Elemental Shaman.